1. Field of the Disclosure
This disclosure relates generally to drill bits that include sensors for providing measurements relating to naturally occurring gamma ray sources.
2. Brief Description of the Related Art
Oil wells (wellbores) are usually drilled with a drill string that includes a tubular member having a drilling assembly (also referred to as the bottom hole assembly or “BHA”) with a drill bit attached to the bottom end thereof. The drill bit is rotated to disintegrate the earth formations to drill the wellbore. The BHA includes devices and sensors for providing information about a variety of parameters relating to the drilling operations (drilling parameters), behavior of the BHA (BHA parameters) and formation surrounding the wellbore being drilled (formation parameters). Drilling parameters include weight-on-bit (“WOB”), rotational speed (revolutions per minute or “RPM”) of the drill bit and BHA, rate of penetration (“ROP”) of the drill bit into the formation, and flow rate of the drilling fluid through the drill string. The BHA parameters typically include torque, whirl and stick-slip. Formation parameters include the various characteristics of the formation, such as resistivity, porosity and permeability.
Information relating to the lithology of a formation may be use useful in several aspects of wellbore construction. In many instances, wellbores are formed along predetermined paths and may intersect a variety of formations. During drilling, a driller may control the drilling parameters such as weight on bit, drilling fluid flow through the drill pipe, drill string rotational speed and drilling mud characteristics. The downhole operating conditions can be dynamic and drilling parameter may require adjustments to efficiently and cost-effectively drill the formation. Knowledge of the formation may be one factor used to adjust these drilling parameters. Also, it may be desirable to drill a wellbore at a specified distance from fluid contacts within the reservoir or from bed boundaries defining the top of a reservoir. Thus, knowledge of the lithology of the formation may be useful in appropriate placing such a wellbore.
Therefore, there is a need for devices, systems and methods for evaluating formations during drilling of a wellbore.